Hello,

I have an analog ammeter 3A full scale.
I want to convert it to 100mA full scale.
I know how to extend the range but how to decrease it?

 

That black coil-like wire between the two terminals is of low resistance and extends the (current) range of the meter.

Getting rid of it or replacing it with the right value resistance will get you the desired range.

 

Get rid of the internal shunt. Determine the sensistivity of the intrinsic meter movement (likely ~50mV or 1mA full-scale). Then calculate new shunt based on the meter sensitivity.

 

Here is a pretty good tutorial on determining the rights value.

http://www.neatcircuits.com/meter-mult.htm

 

I think I'd use 300mA full scale if you plan to keep the same back plate. You'll remember it now but the next guy won't, and that next guy is probably you in a few years.

 

You can make up a new meter face here.

http://www.tonnesoftware.com/meter2.html